1. Field of Invention
The present invention relates to liquid crystal display devices and to electronic apparatuses. In particular, the invention relates to a structure of transflective liquid crystal display devices having excellent visibility, and which can display sufficiently bright images not only in a reflective display mode but also in a transmissive display mode.
2. Description of Related Art
Reflective liquid crystal display devices reduce power consumption because they do not have any light sources, such as a backlight. These devices have therefore been used for various portable electronic apparatuses or the like. However, since the reflective liquid crystal display devices use outside light, such as natural light or illumination light to display images, it is difficult to view the images in dark places. Accordingly, another type of liquid crystal display device has been proposed which uses outside light in bright places, as in the conventional reflective liquid crystal display devices, and uses an internal light source to make displayed images visible in dark places. Hence, this type of liquid crystal display device uses a display system serving as both a reflective display and a transmissive display. In this system, a reflective display mode or a transmissive display mode is selected according to the ambient brightness, thereby displaying clear images even in dark places while reducing power consumption. Hereinafter, this type of liquid crystal display device is referred to as a xe2x80x9ctransflective liquid crystal display devicexe2x80x9d, in this application.
A transflective liquid crystal display device has been proposed which has a reflecting layer having slits (apertures) to transmit light and formed of a metal, such as aluminum on the inner surface of a lower substrate (hereinafter, the liquid crystal side surfaces of substrates is referred to as xe2x80x9cinner surfacesxe2x80x9d, and the opposite surfaces are referred to as xe2x80x9couter surfacesxe2x80x9d) and which allows the reflecting layer to serve as a transflective layer. In this liquid crystal display device, by providing the metallic film on the inner surface of the lower substrate, the parallax effect due to the thickness of the lower substrate is reduced. In particular, color mixture is prevented in a structure having color filters.
FIG. 13 shows an example of a related art transflective liquid crystal display device using the transflective layer.
The liquid crystal display device 100 has liquid crystal 103 held between a pair of transparent substrates 101 and 102. A reflecting layer 104 and an insulating layer 106 are deposited on the lower substrate 101. A lower electrode 108 is formed of a transparent conductive film, such as an indium tin oxide (hereinafter referred to as ITO) film, on the insulating layer 106, and the lower electrode 108 is covered with an alignment layer 107. On the other hand, on the upper substrate 102, a color filter 109 including R (red), G (green), and B (blue) pigmented films are formed. A planarizing layer 111 is deposited on the color filter 109. Upper electrodes 112 are formed of a transparent conductive film, such as an ITO film, on the planarizing film 111, and the upper electrodes 112 are covered with an alignment layer 113.
The reflecting layer 104 is formed of a metal having a high optical reflectivity, such as aluminum, and the reflecting layer 104 has a slit 110 to transmit light in each pixel. The slits 110 allow the reflecting layer 104 to serve as a xe2x80x9ctransflective layerxe2x80x9d (therefore, hereinafter, the reflecting layer 104 is referred to as a transflective layer). In addition, the upper substrate 102 is provided with a front diffuser 118, a retardation layer 119, and an upper polarizer 114 on the outer surface thereof, in that order from the upper substrate 102 side. The lower substrate 101 is provided with a quarter wave plate 115 and a lower polarizer 116 on the outer surface thereof, in that order. Also, a backlight 117 (illumination device) is disposed under the lower surface of the lower substrate 101, that is, under the lower polarizer 116.
When the liquid crystal display device 100 shown in FIG. 13 is used in a reflective mode in a bright place, outside light entering through the upper substrate 102 from above, such as sunlight or illumination light, passes through the liquid crystal 103 to be reflected at the surface of the transflective layer 104 on the lower substrate 101, and then passes through the liquid crystal 103 again to be emitted to the upper substrate 102 side. On the other hand, when the liquid crystal display device 100 is used in a transmissive mode in a dark place, light emitted from the backlight 117 under the lower substrate 101 passes through the slits 110 of the reflecting layer 104, and then passes through the liquid crystal 103 to be emitted to the upper substrate 102 side. This light contributes to displaying images in the respective modes.
In such a transflective liquid crystal display device, a metallic film having a high optical reflectivity, such as an aluminum or silver film, can be used as the reflecting layer. On the other hand, a dielectric mirror formed by alternately laminating dielectric thin layers having different refractive indexes, a cholesteric reflector using a cholesteric liquid crystal, a hologram reflector using a hologram element, can be used. These new types of reflectors not only serve as reflectors to reflect light, making use of the characteristics of the constituents thereof, but also have a particular function.
In particular, the cholesteric liquid crystal exhibits a liquid crystal phase at a specific temperature (liquid crystal transition temperature) or more, in which liquid crystal molecules are arranged in a regular helical manner with a constant pitch. This structure allows the cholesteric liquid crystal to selectively reflect only light having a wavelength corresponding to the pitch of the helix, and thus to transmit the other light. Since the pitch of the helix can be controlled by, for example, changing the ultraviolet light intensity or the temperature when the liquid crystal is hardened, the color of reflected light is locally changeable, and the cholesteric liquid crystal can therefore be used as a reflective color filter. Also, by laminating a plurality of cholesteric liquid crystal layers to selectively reflect light of different colors, the resulting laminate can serve as a reflector to reflect white light.
However, the related art transflective liquid crystal display device as shown in FIG. 13 has a problem in that, while displayed images can be viewed, regardless of the presence or absence of outside light, the brightness of images in a transmissive mode is degraded a lot in comparison with in a reflective mode. This is because, in the transmissive mode, no more than substantially half of light emitted from the backlight is used to display images, and only the light passing through the slits of the transflective layer contributes to displaying images. This problem is also caused by the quarter wave plate and the lower polarizer disposed under the outer surface of the lower substrate and other reasons.
The related art transflective liquid crystal display device has two display modes which are used separately according to whether light is reflected or transmitted. In particular, when the light is transmitted, substantially half of light emitted from the backlight is absorbed by the upper polarizer, and thus only the rest half light is used to display images. Specifically, in a reflective mode, almost all of the linearly polarized light entering from the upper substrate side is used to display bright images. In contrast, in a transmissive mode, the light traveling from the lower surface of the liquid crystal layer toward the upper substrate side must be circularly polarized in order to display images as in the reflective mode. However, half of the circularly polarized light is adsorbed by the upper polarizer while being emitted from the upper substrate to the outside. As a result, no more than substantially half of the light entering the liquid crystal layer can contribute to displaying images. The display principle of this liquid crystal display device itself originally has a factor responsible to make displayed images dark in a transmissive mode.
Also, since the light transmitted through the slits is used to display images in a transmissive mode, the brightness of displayed images depends on the ratio of the entire slit area to the entire transflective layer area (i.e., the aperture ratio). Although a larger aperture ratio enhances the brightness in a transmissive mode, it reduces the area of the transflective layer other than the apertures. Consequently, displayed images become dark in a reflective mode. The aperture ratio of the slits cannot be increased beyond a certain level, in view of ensuring bright images in the reflective mode, and hence it is limited to enhance the brightness in the transmissive mode.
According to the display principle of the transflective liquid crystal display device, since a quarter wave plate is needed under the outer surface of the lower substrate, the brightness becomes insufficient in a transmissive mode. The reason is described below. The following description refers to the structure in which dark images are displayed when a non-selection voltage is applied and bright images are displayed when a selection voltage is applied.
First, when the liquid crystal display device 100 displays dark images in a reflective mode is described with reference to the liquid crystal display device shown in FIG. 13. Light entering from outside of the upper substrate 102 passes through the upper polarizer 114 disposed above the upper substrate 102 to change to linearly polarized light having a polarization axis parallel to FIG. 13 when the polarization axis of the upper polarizer 114 is parallel to the drawing. Then, while passing through the liquid crystal 103, the light changes to substantially circularly polarized light by birefringence of the liquid crystal 103. The light is reflected at the surface of the transflective layer 104 on the lower substrate 101 to reverse the circular polarization direction thereof, and then passes through the liquid crystal 103 again to change to linearly polarized light having a polarization axis perpendicular to the drawing. Thus, the light reaches the upper substrate 102. Since the upper polarizer 114 disposed above the upper substrate 102 has a polarization axis parallel to the drawing, light reflected at the transflective layer 104 is absorbed by the upper polarizer 114, and it does not return to the outside (viewer side) of the liquid crystal display device 100. Thus, the liquid crystal display device 100 displays dark images.
On the other hand, when bright images are displayed in a reflective mode, a voltage applied to the liquid crystal 103 changes the orientation direction of the liquid crystal 103. As a result, after passing through the liquid crystal 103, light entering from the outside of the upper substrate 102 changes to linearly polarized light and is subsequently reflected at the transflective layer 104. The linearly polarized light returns to the outside (viewer side) through the upper polarizer 114 disposed above the upper substrate 102 while maintaining the polarization axis parallel to FIG. 13. Thus, the liquid crystal display device 100 displays bright images.
When the liquid crystal display device 100 displays images in a transmissive mode, light emitted from the backlight 117 enters from the outside of the lower substrate 101 to the liquid crystal 103, and part of the light is transmitted through the slits 110 to contribute to displaying images.
In this instance, in order that the liquid crystal display device 100 displays dark images, the light traveling to the upper substrate 102 through the slits 110 must be circularly polarized as in a reflective mode, as described above. Hence, in order to substantially circularly polarize the light emitted from the backlight 117 and transmitted through the slits 110, the quarter wave plate 115 is required to convert the light linearly polarized by passing through the lower polarizer 116 to circularly polarized light.
As for light not transmitted through the slits 110 in the light emitted from the backlight 117, when the polarization axis of the lower polarizer 116 is perpendicular to FIG. 13, the light emitted from the backlight 117 changes to linearly polarized light perpendicular to FIG. 13 on passing through the lower polarizer 116. Then, it passes through the quarter wave plate 115 to change to substantially circularly polarized light and reaches the transflective layer 104. The light is reflected at the lower surface of the transflective layer 104 to reverse the circular polarization direction, and passes through the quarter wave plate 115 again to change to linearly polarized light having a polarization axis parallel to the drawing. The linearly polarized light is absorbed by the lower polarizer 116 having a polarization axis perpendicular to FIG. 13. In other words, the light emitted from the backlight 117 and not transmitted through the slits 110 is reflected at the lower surface of the transflective layer 104, and is then almost completely absorbed by the lower polarizer 116 under the lower substrate 101.
In the transflective liquid crystal display device 100, as described above, almost all of the light reflected at the transflective layer 104 without passing through the slits 110 in a transmissive mode is absorbed by the lower polarizer 116 under the lower substrate 101, and therefore only part of the light emitted from the backlight 117 can be used to display images. If the light can be transmitted through the lower polarizer 116 to return to the backlight 117 without being absorbed by the lower polarizer 116, the returned light is combined with light originally emitted from the backlight 117, thus enhancing the brightness of the backlight 117 effectively. Consequently, the brightness in a transmissive mode can be enhanced. In other words, if the light reflected at the transflective layer 104 without passing through the slits 110 can be reused to display images, the brightness in the transmissive mode can be enhanced. However, this has not been achieved in the related art structure.
The present invention addresses or solves the problem described above, and provides a transflective liquid crystal display device in which the brightness in a transmissive mode is improved to have excellent visibility. The present invention also provides an electronic apparatus having the liquid crystal display device having the excellent visibility.
In order to address or achieve the above advantages, a liquid crystal display device of the present invention includes a liquid crystal cell having an upper substrate, a lower substrate opposing the upper substrate, and a liquid crystal layer held between the upper substrate and the lower substrate. A color filter layer including a plurality of pigmented films containing different color pigments and a transflective layer including cholesteric liquid crystal films to reflect part of light elliptically polarized light in a predetermined direction and to transmit part of the elliptically polarized light are disposed on the inner surface of the lower substrate, in that order. An upper elliptically polarized light transmitting device to allow elliptically polarized light to enter the liquid crystal layer from above the upper substrate and a lower elliptically polarized light transmitting device to allow elliptically polarized light to enter the liquid crystal layer from below the lower substrate are provided. The liquid crystal layer reverses the polarization of the elliptically polarized light either when a selection voltage is applied or when a non-selection voltage is applied, and does not change the polarization when the other voltage is applied. At least part of the transmission spectrum of each pigmented film included in the color filter layer overlaps with the refection spectrum of the corresponding cholesteric liquid crystal film.
Cholesteric liquid crystal has a so-called selective reflectivity which allows liquid crystal to reflect light circularly polarized in the same direction as the winding direction of the helical liquid crystal molecules and having a wavelength equivalent to the pitch of the helix. Conversely, the cholesteric liquid crystal transmits light having wavelengths that are different from the pitch of the helical molecules therein, and light circularly polarized in the direction opposite to the winding direction of the helical molecules therein even if the light has a wavelength equivalent to the pitch of the helix. Also, the cholesteric liquid crystal films of the present invention do not completely transmit, but partly reflect and partly transmit, light circularly polarized in the same direction as the winding direction of the helical molecules therein and having a wavelength equivalent to the pitch of the helix. This is one of the characteristic features of the present invention. This function allows the cholesteric liquid crystal films to serve as a transflective layer.
The inventors discovered that, in a reflective liquid crystal display device using a reflecting layer formed of cholesteric liquid crystal, which is in the related art, light can be reflected and transmitted in the same display mode by setting liquid crystal to a mode in which light entering a liquid crystal cell is elliptically polarized and in which the polarization of the elliptically polarized light is reversed either when a selection voltage is applied or when a non-selection voltage is applied, and thus images displayed in a transmissive mode do not become dark, according to the display principle. The inventors also discovered that light reflected toward the lower substrate by the selective reflectivity of the cholesteric liquid crystal in a transmissive display mode is reusable even if the structure of the outer surface side of the lower substrate is the same as the related art structure. Focusing attention on these points, the inventors have been reached the present invention.